The present invention relates to well logging apparatus for investigating subsurface formations traversed by a borehole and, more particularly, to apparatus using pad-mounted electrode systems whose active parts operate in the immediate proximity of the borehole walls. Such pad systems are particularly useful in determining the resistivity R.sub.xo of the zone near the borehole likely to be washed by the filtrate of the borehole fluid, better known as the flushed zone.
Various apparatus have been proposed to determine the resistivity of the flushed zone. One of the most commonly used at the present time, referred to as "microspherical focusing system", is described in U.S. Pat. No. 3,798,535, granted on Mar. 19, 1974 to N. A. Schuster. In this system, an array of concentrical rectangular electrodes is mounted on a wide pad sufficiently pliable to conform to the curvature of the borehole wall. A survey current emitted by the central electrode is forced, by appropriately adjusted focusing currents, to flow directly into the formation where it bells out and returns to a remote electrode. The ratio of the potential difference between two potential electrodes to the survey current gives a measurement of the resistivity of a formation zone very close to the borehole wall, which measurement is only slightly influenced by the mudcake resistivity.
The resistivity measurement obtained using this technique is generally good in boreholes which do not present excessive caving and for low contrasts between the resistivity of the mudcake and the resistivity of the flushed zone. However, abnormally low resistivity readings are sometimes obtained in very high contrasts and poor hole conditions. The main reason for such erroneous response has been found to be the poor contact between the borehole wall and such a wide pad. A better application of the pad would require excessively great application forces. Moreover, even if it were possible, by exerting very high application forces, to maintain under most conditions a good contact with the wall, the deformation of the pad would affect the response of the system in an indeterminate manner.
Fast wear is another shortcoming of the present pad. Since the pad must have good flexibility, it is usually made of an elastomer, such as neoprene, which is not rugged enough to withstand repeated friction with the borehole wall. As a consequence, some pads cannot stand more than three or four trips in a well.
The above-discussed problems may be overcome by the use of a narrow rigid pad which can be applied against the borehole wall with higher pressures and, since elastomer is not longer needed, has a good resistance to wear. Unfortunately, a substantial reduction of the width of the present electrode array would result in an excessive reduction of the depth of investigation, making the system almost insensitive to the resistivity of the flushed zone but responsive practically only to the mudcake resistivity.
Thus, it is an object of the present invention to provide a resistivity well logging apparatus using a narrow pad-mounted electrode array capable of measuring the resistivity R.sub.xo of the flushed zone.